This is the fourth in a series of reports presenting particle distribution data acquired in cirrus clouds over the western U.S. The data herein were obtained 21 March 1978 by an instrumented MC-130E near Albuquerque, N.M. The area was under the influence of a slight upper level ridge, and the cirrus appeared related to jet stream winds. No significant surface weather was present at the time. Results include the following: (1) The largest particles measured were near 2500 micrometers, but there were only one or fewer of these per m3; (2) occasional particles as large as 1300 micrometers were recorded in what visually was cloud-free air; (3) few of the particles could be recognized in any known classification system but those most frequently recognized were bullet rosettes; (4) the PMS 2-D data revealed particle-type changes occurring in 5 sec or less of flight time; (5) a halo around the sun was periodically seen when mixed-type crystals containing bullet rosettes were recorded; (6) varying numbers of small particles (the difference between 1 to 28 micrometers) were recorded almost continually, even in clear air, while the aircraft was between 23,000 ft (7.0 km) and 27,300 ft (8.3 km) MSL; (7) computed ice water content values generally decreased with height and increased with temperature in the cirriform clouds sampled; (8) particle size spectra were seldom exponential for particles smaller than approximately 250 micrometers; and (9) bimodal peaks were frequent in the population distribution near 100 and 250 micrometers. (Author).